std: add msghdr_const

As `iovec_const` is to `iovec`, `msghdr_const` is to `msghdr`

std/c/freebsd.zig

@@ -58,6 +58,23 @@ pub const msghdr = extern struct {
     msg_flags: i32,
 };
 
+pub const msghdr_const = extern struct {
+    /// optional address
+    msg_name: ?*const sockaddr,
+    /// size of address
+    msg_namelen: socklen_t,
+    /// scatter/gather array
+    msg_iov: [*]iovec_const,
+    /// # elements in msg_iov
+    msg_iovlen: i32,
+    /// ancillary data
+    msg_control: ?*c_void,
+    /// ancillary data buffer len
+    msg_controllen: socklen_t,
+    /// flags on received message
+    msg_flags: i32,
+};
+
 pub const Stat = extern struct {
     dev: u64,
     ino: u64,

std/c/netbsd.zig

@@ -58,6 +58,23 @@ pub const msghdr = extern struct {
     msg_flags: i32,
 };
 
+pub const msghdr_const = extern struct {
+    /// optional address
+    msg_name: ?*const sockaddr,
+    /// size of address
+    msg_namelen: socklen_t,
+    /// scatter/gather array
+    msg_iov: [*]iovec_const,
+    /// # elements in msg_iov
+    msg_iovlen: i32,
+    /// ancillary data
+    msg_control: ?*c_void,
+    /// ancillary data buffer len
+    msg_controllen: socklen_t,
+    /// flags on received message
+    msg_flags: i32,
+};
+
 pub const Stat = extern struct {
     dev: u64,
     mode: u32,

std/os/linux.zig

@@ -1213,7 +1213,7 @@ pub fn getsockopt(fd: i32, level: u32, optname: u32, noalias optval: [*]u8, noal
     return syscall5(SYS_getsockopt, @bitCast(usize, isize(fd)), level, optname, @ptrToInt(optval), @ptrToInt(optlen));
 }
 
-pub fn sendmsg(fd: i32, msg: *const msghdr, flags: u32) usize {
+pub fn sendmsg(fd: i32, msg: *msghdr_const, flags: u32) usize {
     return syscall3(SYS_sendmsg, @bitCast(usize, isize(fd)), @ptrToInt(msg), flags);
 }
 

std/os/linux/arm64.zig

@@ -2,6 +2,7 @@ const std = @import("../../std.zig");
 const linux = std.os.linux;
 const socklen_t = linux.socklen_t;
 const iovec = linux.iovec;
+const iovec_const = linux.iovec_const;
 
 pub const SYS_io_setup = 0;
 pub const SYS_io_destroy = 1;
@@ -426,6 +427,18 @@ pub const msghdr = extern struct {
     msg_flags: i32,
 };
 
+pub const msghdr_const = extern struct {
+    msg_name: ?*const sockaddr,
+    msg_namelen: socklen_t,
+    msg_iov: [*]iovec_const,
+    msg_iovlen: i32,
+    __pad1: i32,
+    msg_control: ?*c_void,
+    msg_controllen: socklen_t,
+    __pad2: socklen_t,
+    msg_flags: i32,
+};
+
 /// Renamed to Stat to not conflict with the stat function.
 pub const Stat = extern struct {
     dev: u64,

std/os/linux/x86_64.zig

@@ -3,6 +3,7 @@ const linux = std.os.linux;
 const sockaddr = linux.sockaddr;
 const socklen_t = linux.socklen_t;
 const iovec = linux.iovec;
+const iovec_const = linux.iovec_const;
 
 pub const SYS_read = 0;
 pub const SYS_write = 1;
@@ -495,6 +496,18 @@ pub const msghdr = extern struct {
     msg_flags: i32,
 };
 
+pub const msghdr_const = extern struct {
+    msg_name: ?*const sockaddr,
+    msg_namelen: socklen_t,
+    msg_iov: [*]iovec_const,
+    msg_iovlen: i32,
+    __pad1: i32,
+    msg_control: ?*c_void,
+    msg_controllen: socklen_t,
+    __pad2: socklen_t,
+    msg_flags: i32,
+};
+
 /// Renamed to Stat to not conflict with the stat function.
 pub const Stat = extern struct {
     dev: u64,